In recent years, among display devices used for computers and televisions, plasma display panels (hereinafter referred to as “PDPs”) are attracting attention as display devices that can provide a large screen while still being thin and lightweight.
PDPs are gas discharge panels in which images are displayed according to phosphors that emit light by being excited by ultraviolet (UV) radiation generated by a gas discharge. PDPs are divided into two types according to a method used to induce the discharge: AC (alternating current) PDPs and DC (direct current) PDPs, of which the former has especially become mainstream PDPs today because of their superiority over DC PDPs in terms of luminance, luminous efficiency and lifetime.
The structure of a common AC PDP is disclosed, for example, in Patent Reference 1 given below.
To be specific, a common AC PDP has a structure in which a front plate and a back plate arranged so as to oppose each other, and are sealed together at peripheral edges of the panels with sealing glass.
The front plate includes a front glass substrate that has display electrodes in a stripe formation disposed on one main surface thereof, and a dielectric layer formed on top of the display electrodes.
On the other hand, the back plate includes a back glass substrate that has address electrodes in a stripe formation disposed on one main surface thereof, the dielectric layer formed on top of the address electrodes, and successively a protective layer is formed on top of the dielectric layer. Barrier ribs are respectively formed between each two adjacent address electrodes, and phosphor layers are respectively formed between the adjacent barrier ribs after being formed.
The back plate and the front plate are arranged with their respective main surfaces opposing each other so that the electrodes formed on each of the plates are positioned perpendicular to each other. The peripheral edges of the back and front plates are sealed together to form an enclosed space therebetween which is filled with a discharge gas.
Each of the display electrodes is made up of a pair of electrodes with one referred to as an x-electrode and the other as a y-electrode.
An area where a pair of the display electrodes three-dimensionally crosses one address electrode over the discharge space corresponds to a cell that contributes to image display.
The protective layer that covers the dielectric layer formed on the panel glass on the front side of the PDP is formed to protect the dielectric layer from ion bombardment during discharge, and also functions as a cathode electrode that contacts the discharge space. Accordingly, it is noted that the properties of the protective layer exert a profound effect on discharge characteristics.
When a gas discharge is to be produced, first, electrons are emitted from the protective layer, which triggers the gas discharge.
Patent Reference 1 discusses that MgO, which is commonly used as a material for protective layers, is an ideal constituent of protective layers because of the high resistance to sputtering, and also indicates that the use of MgO lowers firing voltage Vf due to the high secondary electron emission coefficient of MgO.
Protective layers made from MgO are usually formed in a thickness of approximately 0.5 μm to 1 μm by vacuum deposition.
Recent years, an advanced television so-called HDTV (High-Definition Television) has progressively come into wide use in which image quality is improved by increasing the number of the scanning lines to be more than that of current television systems.
A standard NTSC system today widely used in Japan and North America has 525 scanning lines, however, an advanced television uses as many as 1125 or 1250 scanning lines.
In terms of PDPs also, there are great hopes for the development of ones having higher luminance and higher efficiency in order to realize high-definition image display as described above.
As the most effective method for achieving high luminance and high efficiency of PDPs, Nonpatent Reference 1 given below, for example, suggests increasing Xe partial pressure in the discharge gas.
The reason comes from the fact that increasing the Xe partial pressure in the discharge gas leads to producing a larger amount of UV radiation emitted when Xe returns to the ground state from an excited state.    [Patent Reference 1] Japanese Laid-Open Patent Application Publication No. H9-92133    [Nonpatent Reference 1] High Efficacy PDP, SID '03 Digest, p. 28